1. Field of the Invention
This invention relates to precision measurements of surface topography, and specifically relates to profilometry and related measurement techniques utilizing contact probing of a surface to measure topographical features on a sub-nanometer scale, through measurement of the change in distance between two mirrors of a multi-pass resonant cavity that is referenced to a frequency stabilized laser source.
2. Summary of the Prior Art
Some of the hardware useful in implementing this invention has already been described in a high-precision alignment system disclosed in U.S. Pat. No. 4,991,962, granted Feb. 12, 1991, to K. Jain, one of the co-inventors of this patent application. The high-precision alignment system of U.S. Pat. No. 4,991,962 features optical and electro-mechanical feedback positioning mechanisms. This configuration includes a Fabry-Perot resonator, with associated electro-optical components, in a system which compares two laser frequencies, by an optical heterodyne technique, to eliminate lateral displacement of mask and sample in an imaging environment. The cavity spacing in the Fabry-Perot resonator is monitored to provide a feedback signal which is used to reposition a mask in exact alignment with a wafer.